Spatial selectivity of stereoscopic mechanisms in human vision that process disparity for the perception of depth will be compared to that of mechanisms that control horizontal disparity vergence. The proposed studies will investigate sustained and transient components of stereopsis and vergence. Oculomotor and psychophysical experiments will examine the orientation and spatial frequency specificity and contrast polarity required for dichoptic stimuli of transient and sustained vergence and stereo perception. The results will be used to formulate a model of disparity processing for sensory and motor systems that may be applied to the analysis of vergence and stereo anomalies as well as to interactions between sustained and transient vergence responses within complex natural visual scenes. Sustained and transient disparity vergence responses will be elicited with two paradigms. In the sustained paradigm, strength of a sinusoidally varying disparity stimulus is measured by the extent to which it can elicit a vergence response from a subject attempting to fixate steadily. The gain and phase of continuous, involuntary sinusoidal vergence tracking is measured as a function of the luminance contrast, spatial frequency, interocular spatial frequency difference and orientation difference of the stereogram stimuli. This involuntary tracking paradigm provides an objective assessment of visual system responses to threshold and suprathreshold stimuli, requiring only that the subject attempt to fixate steadily. The transient paradigm is equivalent to a forced choice procedure in which there are two possible binocular matches of the step disparity stimuli, one that produces a crossed disparity and the other an uncrossed disparity. The initial velocity and latency of step vergence responses is measured to assess the relative strength of the two stimuli. These results and those of comparable measures of stereo threshold will be used to assess the spatial selectivity of binocular sensory and motor functions and to determine if a common source of disparity information is utilized b these binocular sensor and motor function.